The present invention relates to rotation sensors and, more particularly, to a thin film travelling wave rotation sensor.
Navigational and inertial guidance systems for many type of craft often use data about the angular rate of motion of the craft to control the desired movement of the craft. One device which provides data about angular motion is the well known gyroscope. Gyroscopes, however, have many disadvantages. They are physically large and heavy, yet they must be built to extremely high accuracies at great costs, and they may be damaged by even low levels of shock and vibration. To minimize the effects of shock and vibration, they must be protected with heavy mounting devices, thus further increasing size, weight and cost. Furthermore, since critical moveable elements, such as bearings, usually wear with use, the gyroscopes must be frequently maintained to retain precision. Despite the frequent maintenance and high accuracy required, they still may have an error drift rate of fractions of a degree per hour.
Another type of angular motion sensor which attempts to overcome the disadvantages of traditional gyroscopes is disclosed in U.S. Pat. No. 4,899,587, issued to Juergen H. Staudte. That patent discloses an angular rate sensor comprising first and second tuning forks made of quartz. The stems of the forks are coupled together end to end along an axis of symmetry so that the tines face away from each other and lie in a plane. A mount is provided for attaching the dual fork structure to a support. Energy is provided to the tines of the first fork from a pair of electrodes coupled to an oscillator. The oscillator signals cause the tines of the first fork to vibrate in the plane. When the structure rotates above the axis of symmetry, a Coriolis force causes the tines of the second fork to vibrate in a direction normal to the plane. The vibratory motion of the tines of the second fork is sensed with output electrodes for providing a signal indicating angular motion about the single axis.
Unfortunately, the electronics required for driving and sensing the vibratory motion of the forks is very complex, and the output signals are difficult to extract. The device is extremely susceptible to acoustic and vibrational interference, and the piezoelectric properties of quartz make the device very sensitive to stray capacitances. The support mounting for the fork structure creates unwanted stresses and points of failure, and the temperature anomalies of quartz create other electrical and mechanical difficulties. Finally, each device can sense rotation along only a single axis.
U.S. patent application Ser. No. 07/751,280 entitled "Rotation Sensor" was filed by the present inventor on Aug. 29, 1991, and it is incorporated herein by reference. That application discloses a "travelling wave" rotation sensor capable of sensing rotation about two axes wherein the complete structure may be micromachined from a silicon substrate using well known photolithographic and etching techniques. In one embodiment of that invention, a silicon chip is micromachined so that a plurality of tines extend radially and coplanarly from an outer peripheral surface of a circular base. First and second mounting members extend from the base for mounting the base to a support. The first mounting member is disposed generally perpendicularly to the second mounting member. A pulse generator applies electrostatic or electromagnetic pulses to successive free ends of the plurality of tines so that each tine momentarily vibrates in the plane in rotational sequence along the circumference of the base. The successive vibrations establish a net angular momentum approximating a spinning wheel so that rotation of the sensor about the axis of one of the first or second mounting members causes the other mounting member to experience a deformation due to Coriolis force. A plurality of piezoresistive strain gages interconnected to form Wheatstone bridges are disposed on the first and second mounting members for sensing deformation of the first and second mounting members and thus providing signals representative of angular rotation about the first and second axes.